Answer is: D) different room temperatures.
If students want only to measure influence of darkness to fruit, they should not change temperature, because now they will have two different impacts on on fruit spoilness.
The two main variables in an experiment are the independent (in this example should only be darkness) and dependent (spoilness of the fruit) variable.
Dependent variable is the variable being tested and measured in a scientific experiment.
Dependent variables depend on the values of independent variables. The dependent variables represent the output or outcome whose variation is being studied.